1. Field of the Invention
This invention relates generally to loudspeaker systems and in particular relates to an improved loudspeaker having a unique port or vent geometry together with a corresponding method of porting a loudspeaker in an efficient manner and with a novel appearance.
2. Related Art
Vented box loudspeaker systems have been popular for at least 50 years as a means of obtaining greater low frequency efficiency from a given cabinet volume. Significant advances were made in understanding and analyzing vented loudspeaker systems through the work of Thiele and Small during the 1970's. Since then, readily available computer programs have made it possible to easily optimize vented loudspeaker designs. However, practical considerations often prevent these designs, optimized in theory, from being realized in actuality or from functioning as intended.
There are two basic approaches in common use in connection with vented loudspeaker systems, these being the ducted port and the passive radiator. Although the passive radiator approach has some advantages, the ducted port has been, in general, more popular due to lower cost, ease of implementation and generally requiring less space.
There are, however, disadvantages to the ducted port approach. These relate principally to undesirable noise and attendant losses which may be generated by the port at the higher volume velocity of air movement required to produce higher low frequency sound pressure levels. For example, as is well known to those skilled in the art, a vented loudspeaker system has a specific tuning frequency, fp, determined by the volume of air in the enclosure and the acoustic mass of air provided by the port according to the relationship;
  fp  :=            1                        2          ·          π                ⁢                              MAP            ·            CAB                                ·    Hz  where MAP is the acoustic mass of the port and CAB is the compliance of the air in the enclosure. In general, a lower tuning frequency is desirable for higher performance loudspeaker systems. As can be seen, either greater acoustic mass in the port or greater compliance resulting from a larger enclosure volume is required to achieve a lower tuning frequency. The acoustic mass of a port is directly related to the mass of air contained within the port but inversely related to the cross-sectional area of the port. This suggests that to achieve a lower tuning frequency a longer port with smaller cross-sectional area should be used. However a small cross-section is in conflict with the larger volume velocities of air required to reproduce higher sound pressure levels at lower frequencies. For example, if the diameter of a port is too small or is otherwise improperly designed, non-linear behavior such as chuffing or port-noise due to air turbulence can result in audible distortions and loss of efficiency at low frequencies particularly at higher levels of operation. In addition, viscous drag from air movement in the port can result in additional loss of efficiency at lower frequencies. Increasing the cross-sectional area of a port can reduce turbulence and loss but the length of the port must be increased proportionally to maintain the proper acoustic mass for a given tuning frequency. The required increase in length, however, may be impractical to implement. Other difficulties may also arise as the length of the port and cross-section are increased. Organ pipe resonances occur in open-ended ducts at a frequency which is inversely proportional to the length of the duct. These organ pipe resonances may produce easily audible distortion when they occur within certain ranges of frequencies. For example a duct nine inches in length will have a highly audible principle resonance at approximately 700 Hz while a duct only 3 inches in length would have a much less audible principle resonance at approximately 2,100 Hz. In fact, a typical strategy employed in the design of vented loudspeaker systems is the use of shorter ports such that the organ pipe resonances occur at higher frequencies where they are less audible and less likely to be within the range of the transducers mounted in the enclosure. In addition, a larger cross-sectional area may lead to undesirable transmission of mid-range frequencies generated inside the enclosure to the outside of the enclosure. This may also lead to audible distortion in the form of frequency response variations due to interference with the direct sound produced by the loudspeaker system.
Therefore, the design of ports for vented loudspeaker systems involves conflicting requirements. A large cross-sectional area is required to avoid audible noise and losses due to non-linear turbulent flow but this makes it difficult to achieve the acoustic mass required for a low tuning frequency within practical size constraints. As will be familiar to those skilled in the art, various methods have been employed to construct ports with reduced turbulence and loss. One such example is shown in FIG. 1, which is a cross-sectional view of a loudspeaker enclosure 100 including a transducer 102 and a port 104 that is flared at one or both ends of the port in order to reduce turbulence. The flared port 104 operates to reduce turbulence by increasing the cross-sectional area of the port at one or both ends thereby slowing the particle velocity of air at the exits. This allows for a smaller cross-section in the middle section of the port and a higher acoustic mass for a given length. However, in order to be effective, the required flared ends 106, 108 may be quite large and may, themselves, add significantly to the overall port length without significantly contributing to the acoustic mass. The increased cross-section of the flare may increase the transmission of undesirable midrange frequencies from inside the loudspeaker cabinet and an improperly selected rate of flare may actually increase turbulence.
Another conventional method used to decrease turbulence and loss is shown in FIG. 2, which is a cross-sectional view of a loudspeaker enclosure 200 with a transducer 102 and multiple ports 204 and 206. Using multiple ports 204 and 206 decreases turbulence and loss by taking advantage of the combined cross-sectional area of several ports. However, as with a single port, the length of each of the multiple ports must be increased to account for the greater total cross-section. For example, if two identical ports are used they will both need to be approximately twice as long as a single port of the same cross-section to achieve the same acoustic mass and tuning frequency. As discussed above this may lead to impractical length requirements and more audible organ pipe resonances.
Other techniques are also used to reduce turbulence and loss as well as the other difficulties associated with the design of ports as previously discussed. These include ports with rounded or flanged ends, geometries to reduce organ pipe resonances and a plethora of methods for implementing longer ports through folding or other convolutions.
U.S. Pat. Nos. 5,517,573 and 5,809,154 to Polk, et al., incorporated herein in their entirety by reference, disclose improved porting methods for achieving the required acoustic mass in a compact space with reduced turbulence and loss. FIG. 3 is a reproduction of FIG. 7 from the '573 patent. The method described in these patents involves the use of a disk at the end or ends of a simple duct to effectively create an increasing cross-sectional area at the ends of the port. In some preferred embodiments flow guides are also used to further improve the efficiency of the port structure. This method has the advantages of suppressing transmission of midrange frequencies from inside the cabinet and of providing the required acoustic mass in a more compact form which also reduces turbulence and loss.